Abstract
The speciation and fate of neptunium as Np(V)O2(+) during the crystallization of ferrihydrite to hematite and goethite was explored in a range of systems. Adsorption of NpO2(+) to iron(III) (oxyhydr)oxide phases was reversible and, for ferrihydrite, occurred through the formation of mononuclear bidentate surface complexes. By contrast, chemical extractions and X-ray absorption spectroscopy (XAS) analyses showed the incorporation of Np(V) into the structure of hematite during its crystallization from ferrihydrite (pH 10.5). This occurred through direct replacement of octahedrally coordinated Fe(III) by Np(V) in neptunate-like coordination. Subsequent analyses on mixed goethite and hematite crystallization products (pH 9.5 and 11) showed that Np(V) was incorporated during crystallization. Conversely, there was limited evidence for Np(V) incorporation during goethite crystallization at the extreme pH of 13.3. This is likely due to the formation of a Np(V) hydroxide precipitate preventing incorporation into the goethite particles. Overall these data highlight the complex behavior of Np(V) during the crystallization of iron(III) (oxyhydr)oxides, and demonstrate clear evidence for neptunium incorporation into environmentally important mineral phases. This extends our knowledge of the range of geochemical conditions under which there is potential for long-term immobilization of radiotoxic Np in natural and engineered environments.
Highlights
Many nations have substantial nuclear legacy wastes due to their civil and military nuclear programs.[1−7] These materials contain significant levels of Np-237, an α-emitting radionuclide with a long half-life (2.14 × 106 y) which is highly radiotoxic and will be a significant dose contributing radionuclide in radioactive wastes on longer time scales.[5−8] Understanding the environmental behavior of Np is a key consideration at nuclear legacy sites such as Sellafield[4] and Hanford,[2,3] and for deep geological disposal of radioactive wastes.[5,6]
For Np(V), the thermodynamic solubility decreases from ∼100 μM at pH 7 to ∼0.1 μM at pH 10.5 With this in mind, we have focused on alkaline pH conditions to allow specific insights into the fate of Np(V) during the transformation of ferrihydrite to crystalline ironoxide phases
The results from the X-ray diffraction (XRD) and transmission electron microscopy (TEM) analyses are summarized in Table 1 and Figures SI1 and SI2
Summary
Due to the reduced solubility of metal ions/radionuclides at elevated pH, base addition has been used to treat radioactive wastes,[14] and cementitious materials have been included in the engineering of geological disposal scenarios with the intention that this will increase pH and decrease the solubility and mobility of radionuclides.[15] For Np(V), the thermodynamic solubility decreases from ∼100 μM at pH 7 to ∼0.1 μM at pH 10.5 With this in mind, we have focused on alkaline pH conditions (pH > 9) to allow specific insights into the fate of Np(V) during the transformation of ferrihydrite to crystalline iron (oxyhydr)oxide phases. Iron(III) (oxyhydr)oxide phases such as ferrihydrite (Fe9.74O14(OH)2),[16] hematite (Fe2O3)[17] and goethite (FeOOH)[18] are ubiquitous throughout the environment and Received: November 12, 2015 Revised: February 19, 2016 Accepted: February 25, 2016 Published: February 25, 2016
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